REVIEW Open Access Hierarchical micro/nano structures for ... · in a variety of self-cleaning surface applications such as optical component, lens, displays, electronic equipment,

Yoon et al. Micro and Nano Systems Letters 2014, 2:3http://www.mnsl-journal.com/content/2/1/3

REVIEW Open Access

Hierarchical micro/nano structures forsuper-hydrophobic surfaces and super-lyophobicsurface against liquid metalYoungsam Yoon*, Daeyoung Kim and Jeong-Bong Lee

Abstract

Non-wetting super-hydrophobic or super-lyophobic surfaces are of great interest in a variety of applications. Naturalwater repelling surfaces show micro-/nano- combined hierarchical structure which shows extremely low wettabilityand self-cleaning characteristics. Inspired by such natural wonders, there have been tremendous efforts to createartificial non-wetting super-hydrophobic or super-lyophobic surfaces. In this paper, recent progress in artificialsuper-hydrophobic surfaces based on hierarchical micro-/nano- dual-scale structure is reviewed. In addition, applicationof hierarchical micro-/nano- dual-scale structure as super-lyophobic surfaces against gallium-based liquid metal alloy isalso reviewed.

Keywords: Super-hydrophobic; Super-lyophobic; Lotus effect; Self-cleaning; Hierarchical structure; Liquid metal;Galinstan®

ReviewNatural water repelling surfaces, such as lotus leaves,butterfly wings, and shark scales exhibit high water con-tact angle in excess of approximately 150° [1–3]. Surfaceswith water contact angles greater than 150° is calledsuper-hydrophobic. It has been reported that such super-hydrophobicity stems from a combination of hierarchicalmicro-/nano-scale surface texturing and surface chemicalcomposition [4]. Combination of micro- and nano-scalestructure is crucial as water droplet on these hierarchicalstructures may touch on the apex of the nano-scalestructures without fully sitting on the surface because airpockets fill in the vicinity of nano-scale structures beneaththe droplet resulting in low contact area of water droplet,high contact angle, low contact hysteresis, and low adhesiveforce [5].Mimicking such natural wonders and creating artificial

super-hydrophobic surfaces on various solid substrateshas been of great interest over the past few decades andthere have been considerable amount of reported litera-tures on the topic. Although a variety of materials andtechnologies were studied to create artificial hierarchical

* Correspondence: [email protected] of Electrical Engineering, The University of Texas at Dallas, Richardson,TX, USA

© 2014 Yoon et al.; licensee Springer. This is anAttribution License (http://creativecommons.orin any medium, provided the original work is p

micro- /nano-scale surface textures for super-hydrophobicity,many of the studies are based on materials that are solidand optically opaque in visible wavelengths. Super-hydrophobic surfaces with high optical transmission invisible wavelengths have considerable potential to be usedin a variety of self-cleaning surface applications such asoptical component, lens, displays, electronic equipment,automobile glass, anti-fog consumer glass, solar panels,among others. In order to maintain high optical transpar-ency, it is critical that structural roughness on the artificiallotus effect surface should be much smaller than the wave-length of transmitted light [6].Along with super-hydrophobic surfaces, there have been

great interests in creating anti-wetting surfaces against oils[7] as well as microfluidic platforms for manipulatinggallium-based non-toxic liquid metals [8]. With thephenomenal advance of artificial super-hydrophobic sur-faces, hierarchical micro-/nano-scale surfaces may providestarting platforms for study on creating super-oleophobicsurfaces and super-lyophobic surfaces against liquidmetals.This review discusses recent research progress on

hierarchical micro-/nano-scale surfaces as artificial super-hydrophobic surfaces as well as super-lyophobic surfacesagainst gallium-based liquid metal alloys.

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Theoretical backgroundsDefinition of lyophobic/lyophilicThe definition of ‘lyophobic/lyophilic’ is the physicalproperty of a molecule that is repelled from or attractedto a mass of liquid. Therefore, the ‘lyophobic or lyophilic’should be used with indication of target liquid excepthydrophobic/hydrophilic or oleophobic/oleophilic thatimplies target liquid is water or oil, respectively. The‘lyophobic/lyophilic’ is the superordinate concept tohydrophobic/hydrophilic.In order to characterize lyophobicity (either lyophobic

or lyophilic), static and dynamic contact angles are im-portant parameters. Static contact angles can be measuredby using a contact angle goniometer which can capturethe profile of a liquid droplet on a solid substrate. Idealstatic contact angle depends on the surface tensions ofliquid (liquid/gas interfacial energy) and solid (solid/gasinterfacial energy) and liquid/solid interactions. As shownin Figure 1a, the static contact angle is defined as the angleformed by the intersection of the liquid–solid interfaceand the liquid–gas interface.Generally, when the contact angle is smaller than 90°,

the solid surface is considered to be ‘lyophilic’ whichmeans wetting of liquid on the surface is favorable. Whenthe contact angle is larger than 90°, the solid surface isconsidered ‘lyophobic’ which indicates the liquid willminimize its contact on the surface and form a compactliquid droplet. When the contact angle is greater than150°, the solid surface is considered to be ‘super-lyophobic’.And when the contact angle is almost 0°, the surface isconsidered to be ‘super-lyophilic’ [9].

Surface tensionIdeally, as explained, the contact angle of liquid on thesolid is determined by the surface tension of liquid, sur-face energy of solid, and their interaction. In a pure liquid,as shown in Figure 2, molecules in the bulk of liquid haveinteractions with all neighboring molecules so that the netforce should be zero. The interactions are mostly origi-nated from Van der Waals forces for non-polar liquidsand hydrogen bonds for polar liquids such as water. Onthe other hand, molecules exposed at the surface do nothave interactions on all sides of them to provide abalanced net force, resulting in the local asymmetry ofinteractions. The resulting dissymmetry in the interactions

Figure 1 Schematic diagrams of different wetting regimes: (a) Young

causes pulling internal force which is called as the surfacetension. Therefore, surface tension of the liquid changesits geometry to minimize this energy defect, forming aspherical shape. Generally, for a solid substrate, the ‘surfaceenergy’ is used to have equivalent meaning of surfacetension for a liquid.

Wetting modelsAs shown in Figure 1a, Thomas Young described the con-tact angle of a liquid droplet on an ideal solid substrate[9]. It is determined by the thermodynamic equilibrium ofsurface tensions of three interfaces under the droplet:

cosθ ¼ γSG−γSLγLG

ð1Þ

where θ is contact angle and γSG, γSL, and γLG are surfacetension of solid-gas, solid–liquid and liquid–gas interfaces,respectively. Equation (1) is usually referred as Young’sequation. It should be noted that Young’s equation onlyworks with flat homogeneous surface. So, recent advancesin artificial super-hydrophobic surfaces with non-planargeometry should be modeled by modified equations otherthan Young’s equation. On roughened surfaces, there aremainly two different equilibrium states: Wenzel and Cassiestate. As shown in Figure 1b, when a liquid can fully wetthe surface texture, the thermodynamic equilibrium contactangle of a liquid droplet is described by the Wenzel model[10] which is given by:

cosθW ¼ r cosθ ð2Þ

where θw is the apparent contact angle in Wenzel state, θrefers to Young’s contact angle, and r is a roughness factor,which is defined as the ratio of actual area of a roughsurface to the flat, projected area. Droplets in this fully-wetted Wenzel state typically display very high hysteresisbecause the contact line of the droplets becomes severelypinned on surface asperities. On the other hand, when aliquid cannot penetrate into the surface texture, the drop-let forms a highly non-wetting regime known as a Cassie-Baxter state or simply Cassie state (Figure 1c). Because ofa decrease in the effective contact area between a surfaceand a droplet by entrapped air, it can have low adhesiveforce. Therefore, compared to Wenzel state, the droplet inthe Cassie state displays a very high contact angle as well

’s model, (b) Wenzel model, and (c) Cassie-Baxter model.

Figure 2 A schematic diagram of surface tension of liquid dueto asymmetry of interactions at the surface of liquid.


as low hysteresis, resulting in easy roll-off characteristicdue to its low adhesive force between the liquid dropletand the solid substrate. If fs is the fraction of the solid incontact with the liquid, the Cassie equation can be repre-sented by equation (3) [11]:

cosθC ¼ f S 1þ cosθð Þ−1 ð3Þ

where θC is effective contact angle in Cassie state. Thisequation indicates the contact angle increases as the solidfraction decreases in Cassie state. Additionally, in orderto have more accurate predictions of advancing andreceding contact angles, the modified Cassie-Baxter rela-tion which includes a local differential texture parameterwas proposed [12].

Contact angle hysteresisAlong with static contact angle, in order to quantify the‘dynamic’ wetting property of liquid on a solid surface(when the droplet is in the transitional motion), ‘dynamic’contact angle is an important parameter. When a dropletis placed on an inclined surface, the droplet can experi-ence gravitational force so that the shape of the dropletbecomes asymmetric; on the downhill side, the dropletadvances but on the uphill side, the droplet recedes.Therefore, the contact angle of the droplet on the down-hill side is an advancing contact angle and that of thedroplet on the uphill side is a receding contact angle. Thedifference between the advancing angle and the recedingangle is called contact angle hysteresis. Due to the differ-ence of advancing and receding contact angles, the dropletcan stick to the surface against gravitational force. Add-itionally, the dynamic contact angle can be measured bychanging the volume of liquid. The volume of liquid isincreased or decreased until the droplet has a maximumor minimum contact angle without changing the surfacearea between the liquid and the solid substrate. The

maximum and minimum contact angles are called the ad-vancing and receding contact angle, respectively, and theadvancing and receding contact angles are obtained froma series of images from a recorded video just before thecontact line changes. When the advancing and recedingcontact angles of liquid on a solid substrate are close eachother, which means the lower contact angle hysteresis, itindicates the substrate is lyophobic to the liquid [13].

Super-hydrophobic surfacesSuper-hydrophobic surfaces in natureThe hydrophobic property of nature’s Nelumbo nucifera(lotus) has been observed by Barthlott and Neinhuis [14].It was shown that water droplets placed on the apex ofnanostructures combined with microstructures do notwet the surface as the hierarchical structure provides airpockets due to the dual-scale roughness morphology [15].Surface of the lotus showed static contact angle and con-tact angle hysteresis approximately 164° and 3°, respect-ively [16]. Such super-hydrophobicity of the natural lotusleaf surface enables self-cleaning of the leaf surface as theroll-off water droplets collect dusts in the path. As shownin SEM images (Figure 3), it clearly showed micro-scaleelliptic protrusions with an average diameter of approxi-mately 9.4 μm, pitch distance of 12 μm, and structuralheight of approximately 18 μm. In addition, surface of themicro-scale protrusions and bottom surface were thor-oughly covered with random nano-scale textures. Thereare countless examples of natural super-hydrophobicsurfaces in the plants such as taro (Colocasia esculenta),India canna (Canna generalis bailey), and rice leaf [17]. Asshown in Figure 4, the large water contact angle of taroleaf, India canna, and rice leaf were 159 ± 2°, 165 ± 2°, and157 ± 2°, respectively and there were homogeneousconformal structures on the surface in the range of fewnanometer to micrometer which influence the wettabilityof surfaces. The sliding angle of taro leaf is 3° and that ofrice leaf is 4° or12° depending on the direction of slidingof a droplet (parallel and perpendicular direction tosurface papillae) due to anisotropic wetting property.Surface morphology inducing water repelling character-

istics of insect wings were also investigated and it wasagain found that surface of the insect wings have hier-archical micro-/nano-scale surfaces. Figure 5 shows watercontact angle of various insect wings such as HomopteraMeimuna opalifera (Figure 5a, 165°), Orthoptera acridacinerea (Figure 5b, 151°), and Hymenopetra vespa dybow-skii (Figure 5c, 125°) as well as SEM images of hierarchicalstructures which consist of micro-/nano-scale layers onthe upper surfaces of insect wings [1,2]. From the nature’splant leafs and insect wings, it is clear that the hierarchicalmicro-/nano-scale surfaces are essential to demonstratesuper-hydrophobicity.

Figure 3 Natural lotus leaf: (a) an optical image and (b-d) SEM images.


Artificial super-hydrophobic surfacesUnderstanding reasons for the super-hydrophobicity invarious plant surfaces and insect wings among others innature has allowed extensive researches in artificial super-hydrophobic surfaces using various materials. A surface

Figure 4 Various natural super-hydrophobic surfaces: (a) water dropletdroplets on an India canna leaf, (d) SEM image of the India canna leaf ssurface. With permission from Elsevier, Copyright 2007.

with combined characteristics of high water contact angles(>150°) with low sliding angles (tilted angle which allowsfree rolling of water droplets on a surface) typicallysmaller than 10° is commonly known as self-cleaningsurfaces. One of the motivations for the popularity in the

s on a taro leaf, (b) SEM image of the taro leaf surface, (c) waterurface, (e) water droplet on a rice leaf, (f) SEM image of the rice leaf

Figure 5 SEM images of the hierarchical micro-/nano-scale structures of the surface of the wings of various insects: (a, b) HomopteraMeimuna opalifera (Walker), (c, d) Orthoptera Acrida Cinerea cinerea (Thunberg), (e, f) Hymenoptera Vespa dybowskii (Andre). With permissionfrom Elsevier, Copyright 2009.


research community on the super-hydrophobic surfacesis because of potential manufacturing of artificial self-cleaning surfaces. Inspired by natural super-hydrophobicsurfaces, a wide variety of fabrication processes such asdeep reactive ion etch (DRIE) process [18,19], electrode-position [20–22], self-assembly [23], plasma treatment[21,24], chemical vapor deposition [22,25,26], and layer-bylayer deposition [27] have been studied to fabricate artifi-cial biomimetic super-hydrophobic hierarchical micro-/nano-scale structures.Among various artificial super-hydrophobic surfaces,

optically transparent super-hydrophobic surfaces have greatpotential to be used in a variety of self-cleaning surfaceapplications such as the prevention of the adhesion of dustand snow to window glasses, traffic indicators, goggles,and solar panels, etc. Optically transparent coatings, self-cleaning surfaces and anti-reflective coatings are activeresearch topics targeted for the coating industry, consumerglass, optical component/lens manufactures, displays/electronic equipment and aerospace. In order to make sur-faces transparent to certain range of wavelengths of light, itis essential that the surface roughness of the artificial super-hydrophobic surfaces should be much smaller than thewavelength of light [6]. This provides one of the key designcriteria for the optically transparent self-cleaning surfaceswhich should have nano-scale roughness much smallerthan wavelength of visible light (400 ~ 700 nm) whilehaving micro-scale roughness for enhanced air-trapping.Among various hierarchically textured super-hydrophobic

surfaces, in order to introduce various materials andmethods to fabricate them, we have studied four differentsuper-hydrophobic surfaces: silica nanoparticles, carbon

nanotube (CNT), PDMS, and SU-8 based super-hydrophobic surfaces.

Silica nanoparticles based super-hydrophobic thin filmIn order to realize artificial hierarchical micro-/nano- dual-scale surface roughness, silica based raspberry-like nano-particles were investigated. W. Ming et al. showed a super-hydrophobic film from well-defined raspberry-like silicananoparticles (Figure 6) using Stöber method [24,28]. Theamine functionalized raspberry-like particles which weredispersed in ethanol were deposited on the epoxy filmthrough the reaction between amine and epoxy at 75°C.Therefore, loose particles could be flushed away with etha-nol, and then only one layer of the raspberry-like particlescovalently bonded to the epoxy based films. It showed ahigh advancing angle of 165 ± 1° along with a contactangle hysteresis of ~ 2°, and a low sliding angle of 3 ± 1°against 10 μL water droplet on the surface. Although, thisapproach demonstrated a super-hydrophobic surface, itrequired a series of relatively complex process such as thepreparation of amino-functionalized nanoparticles, epoxy-functionalized micro-particles, and a long reaction andseparation procedure.Additionally, with a relatively simple procedure, Qian

et al. also developed the hierarchical dual-scaled raspberry-like particles by introducing poly acrylic acid (PAA) - func-tionalized polystyrene (PS) particles into hydrolysis reactionof tetraethoxysilane (TEOS). As shown in Figure 7, PAA-functionalized PS was mixed with an ethanol solutioncontaining TEOS and ammonia was added to catalyze thehydrolysis reaction of TEOS to form the silica nanoparticlesaround the PS. Then, raspberry particles were easily made

Figure 6 Artificial super-hydrophobic surfaces. (a) TEM image of raspberrylike silica particles and (b) an optical image of water droplet placed onthe surface.


after centrifugation, and it was deposited on the glasssubstrate for hierarchical structured morphology. Aftermodification of the surface with dodecyltrichlorosilane,super-hydrophobic surfaces were produced with a highcontact angle of 162.1° (Figure 8) [29].

Carbon nanotube based super-hydrophobic thin filmBased on utilizing nanoparticles, super-hydrophobic thinfilm can be realized by achieving artificial hierarchical dual-scale surface roughness. Li et al. reported aligned CNT(ACNT) films which had perpendicular nanotubes againstthe substrate showed super-hydrophobic property [26].Due to its structural property, it can have large fraction ofair inducing a low contact area. The grown ACNT films

Figure 7 Schematic diagram of process for preparation of raspberry-likCopyright 2009.

showed contact angle of 158.5 ± 1.5° and it was increased to171 ± 0.5° by immersion in a methanolic solution of hydro-lyzed fluoroalkylsilane. The super-hydrophobic propertyof ACNT was enhanced by having low surface energymaterials. Similarly, Lau et al. also reported that carbonnanotube forest coated with polytetrafluoroethylene (PTFE)showed the stable super-hydrophobic property [30] asshown in Figure 9. The grown CNT forest showed initialstatic contact angle of 161° but the water droplets were notstable and permeated into the CNT forest voids after a fewminutes. However, by depositing PTFE layer onto the CNTforest, the advancing and receding contact angles weremeasured to be 170° and 160° indicating the stable super-hydrophobic property. In addition, Jung and Bhushan

e particulate films. With permission from Journal of Materials Chemistry,

Figure 8 The raspberry like silica nanoparticles: (a) TEM image, (b) Contact angle, and (c) 10 μL water droplet on the surface. The scale bar of(a) indicates 333 nm. With permission from Journal of Materials Chemistry, Copyright 2009.


reported that the CNTs based hierarchical structure createdby spray method and showed a high static contact angle of170° and a low contact angle hysteresis of 2° [31]. TheCNT-based hierarchical dual-scale structure could maintainits super-hydrophobic property even after exposing to waterfalling on it with a pressure of 10 kPa for 24 hours. Thestatic contact angle was remained greater than 150° and thecontact angle hysteresis was maintained less than 15°.

Figure 9 SEM images of carbon nanotube forests. (a) As-grown forest prep(b) PTFE-coated forest after HFCVD treatment, and (c) an essentially sphericalpermission from American Chemical Society. Copyright (2003).

SU-8 based super-hydrophobic thin filmSU-8, one of the most popular materials used in the fieldof microelectromechanical systems (MEMS), shows awater contact angle of 73.1 ± 2.8° and surface energy of45.5 ± 0.3 mJ/m2 [32]. Although the material itself doesnot show hydrophobicity, SU-8 is one of the great mate-rials to create super-hydrophobic thin film because it isreadily fabricated in high aspect ratio microstructures to

ared by PECVD with nanotube diameter of 50 nm and a height of 2 μm,water droplet suspended on the PTFE-coated forest. Reprinted with


have roughened surface which can be tuned its wettingproperty. In order to realize the property, it is essential tocombine artificial nano-scale textures with the micro-patterned SU-8, forming hierarchical micro-/nano-scalestructured surface. Recently, there have been a few effortsto create artificial super-hydrophobic thin film using SU-8as surveyed below.Hong et al. prepared a direct mixture of polytetrafluoro-

ethylene (PTFE) nanoparticles into SU-8, spin-coated thePTFE-SU-8 nano-composite on transparent substratessuch as glass and polymers, and photo defined micro-scale patterns with a minimum feature size of 50 μm [33].This film showed water contact angle of 150°, which issignificant improvement in hydrophobicity from normallynot so hydrophobic SU-8 flat surface. This is attributed tothe effect of formation of hierarchical roughness on thesurface as well as reduction of effective surface energy ofthe substrate. Although this result seemed good, opticaltransparency of this PTFE-SU-8 nano-composite film wasonly 31%, prohibitively low in practical optical trans-parency applications. They also reported an alternativemethod in which the PTFE nanoparticles were spraycoated and thermally immobilized onto the exposed SU-8matrix. During the SU-8 developing process, PTFE nano-particles sprayed onto the unexposed SU-8 layer were alsoremoved. The PTFE-SU-8 film prepared by this alternativemethod showed a water contact angle of 165° ~ 167° andan optical transparency up to 80%.Marquez-Velasco et al. reported super-hydrophobic sur-

faces fabricated in SU-8 by having micro- and nano-scaletopography [34]. In this work, SU-8 was patterned to havelow aspect ratio cylindrical and square pillars with thicknessof 45 μm and 75 μm. Then, the SU-8 pillars were O2

plasma treated to roughen the SU-8 pillar surface to have20 nm (30 sec. etch) or 2 μm (5 min. etch) sub-pillarsformed on the SU-8. It was shown that 2 μm sub-pillarswere too tall and readily clung together to lose its purpose(dual-scale topography). However, dual-scale topographySU-8 formed by 20 nm sub-pillars clearly showed super-hydrophobicity (contact angles > 157°, contact angle hyster-esis as low as 5º). Although there was a potential, opticaltransparency of the dual-scale topography of this SU-8 filmwas not reported.We reported a method to create optically transparent

nano-patterned SU-8 micro-pillar array as a super-hydrophobic thin film [35]. As shown in Figure 10, a densearray of tapered SU-8 micro-pillar array was fabricatedusing previously reported SU-8 backside exposure method[36]. Then, it was isotropically dry etched for 10 min. usingO2/CF4 (90% : 10%) plasma with a power of 300 W ina microwave plasma etcher (TePla PS 300, PVA TePlaAmerica, Inc., Corona, CA, USA). The top and bottomdiameter of the SU-8 micro-pillar was 50 μm and 85 μm,respectively, and the height was 250 μm with a 3° ~ 5°

tapered angle (Figure 11a and b). Isotropic plasma etchingprocess made nano-porous patterns (10 ~ 900 nm)uniformly on the surface of the non-planar micro-pillarstructures (Figure 11c and d). The advancing angle of thethis super-hydrophobic SU-8 thin film was in the range of156° ~ 161° (0.025 μL/sec. dispense rate) and the recedingangle was measured to be 122° ~ 127° (Figure 12). Opticaltransparency was found to be 60 ~ 70% by an n&kanalyzer which shows feasibility of the optically transparentapplications (Figure 13).

PDMS based super-hydrophobic thin filmAlthough SU-8-based artificial super-hydrophobic thinfilms overall showed excellent hydrophobicity, the opticaltransparency in visible wavelengths turned out to be lessthan 80%. For practical optically transparent self-cleaningthin film applications, it is highly desirable to have greaterthan 80% optical transparency. Polydimethylsiloxane(PDMS) was one of the popular materials studied by vari-ous groups for optically transparent artificial self-cleaningthin film applications. The popularity for the PDMS wasdue to many favorable inherent material properties. Someof the key material properties of the PDMS for theoptically transparent super-hydrophobic thin film applica-tions are its low surface energy of 19.8 mJ/cm2 [37], highoptical transparency throughout the ultraviolet and visiblewavelengths [38], and extremely low Young’s modulus(<4 MPa) [39] which makes the PDMS ideal material forflexible self-cleaning super-hydrophobic thin film applica-tion to conform to arbitrary shaped non-planar surfaces.It is also well-known for reliably maintaining dimensionsin the replicated inverse images of a master mold usingsoft lithography [40].Huang et al. demonstrated a dense array of replicated

PDMS micro lens and PDMS micro bowls [41] by singleand double soft lithography, respectively from an array oftall and sharp photoresist spikes using their unique 3Ddiffuser lithography technique [42]. The PDMS microbowl array showed water contact angle of approximately164.6° and low adhesive force, outstanding characteristicfor the self-cleaning thin film applications. Im et al.further demonstrated a robust super-hydrophobic andsuper-oleophobic surface with inverse trapezoidal micro-structures on PDMS [43]. Again, they used their uniquebackside 3D diffuser lithography to fabricate a photoresistmold and PDMS trapezoidal structures were replicatedusing soft lithography method (Figure 14). The inversetransparent and flexible PDMS trapezoidal microstruc-tures have the following dimensions: pitch distance of40 μm, diameter of 26 μm at the top, diameter of 15 μmat the bottom. The contact angle of Teflon coated PDMStrapezoid surface was as high as 153° and it also showedthe high contact angle of 135° using methanol droplet(Figure 15).

Figure 10 Fabrication sequence of the nano-patterned SU-8 micro-pillar array. (a) spin coating, (b) backside exposure, (c) Tapered pillararray formation, and (d) O2/CF4 plasma etch.

Figure 11 SEM images of nano-patterned SU-8 micro-pillar array: (a) after plasma treatment, (b) close-up view, (c) nano-patterned surface,and (d) plasma treated top pillar area. With permission from Journal of Micromechanic Microengineering, Copyright 2012.


Figure 12 Dynamic contact angles of the SU-8 super-hydrophobic thin film. With permission from Journal of MicromechanicMicroengineering, Copyright 2012.


Recently we reported an extremely simple one-stepfabrication method of flexible, optically transparent super-hydrophobic PDMS thin film using under-exposed under-baked photoresist mold [44]. As shown in Figure 16, thisapproach intentionally utilizes significant under softbaking so it allows photoresist retains good amount ofsolvent which in turn greatly increases the dissolution rateof the photoresist during the developing process. Oncethis intentional under baking condition is combined withunder exposure condition, it would create a unique broc-coli like structure with near randomly created nano-scalesurface due to dissolved solvents. A positive PR, AZP4620was severely under soft baked in an 88°C convection ovenand subsequently it was under exposed with the exposure

Figure 13 Optical transmittance for various films on the SU-8 surfaceCopyright 2012.

dose of 800 mJ/cm2. After the photoresist was developedby AZ400K (1:3) developer, broccoli like micro-/nano-scalecombined hierarchical structures was formed inside of thephotoresist mold. Then, it was replicated by PDMS in avacuum oven to completely replicate the extremely concavearea on the surface of the photoresist. After the curing ofPDMS, the photoresist mold was stripped away usingAZ 400 T to safely delaminate the replicated PDMS fromthe mold. Figure 17 shows SEM images of natural lotusleaf compared with replicated PDMS super-hydrophobicsurfaces prepared by this approach. As shown in Figure 18,the water static contact angle was 157.7 ± 1°, 160 ± 1°, and161 ± 0.5° for 4,10, and 15 μL droplets, respectively whichclearly shows super-hydrophobic nature of the PDMS thin

. With permission from Journal of Micromechanic Microengineering,

Figure 14 Unique backside 3D diffuser lithography. (a) A schematic diagram of backside 3D diffuser lithography, (b) SEM image of inversetrapezoid with magnification (50 μm), (c) with magnification (20 μm), (d) with magnification (5 μm).


film. The water contact angles were slightly increased tothe range of 160° ~ 163° by depositing 100 nm fluorocarbon(CxFy) film. The optical transmittance of bare PDMS filmwas in the range of 95 ~ 100%, while that of the broccoli-shaped super-hydrophobic PDMS film was in the range of94 ~ 98% and the fluorocarbon-coated super-hydrophobic

Figure 15 Super-hydrophobic characterizations. (a) Contact angle of Teflsnapshots of high speed camera images of bouncing water droplet.

PDMS film was in the range of 88 ~ 92% at around550 nm wavelength [45]. As a demonstration of an appli-cation, Figure 19 shows an optical image of a solar panelcovered with a super-hydrophobic PDMS film in whichtwo water droplets were placed to show self-cleaningeffect.

on coated trapezoid, (b) contact angle of the methanol, and (c)

Figure 16 Schematic diagrams of the photoresist patterning: (a) conventional lithography, (b) under-exposure condition, and (c) under-exposure under-baking condition. With permission from IEEE MEMS, Copyright 2012.


Super-lyophobic surfaces against liquid metalsLiquid metal is the metal in liquid phase at roomtemperature. As it has liquid property which is continu-ously deformable as well as metallic property such as highthermal and electrical conductivity, it can be applied tovarious applications such as radio frequency (RF) MEMSswitch [46], electro-wetting on dielectrics (EWOD) [47,48],tunable and flexible antennas [49], frequency selectivesurfaces (FSS) [50], and heat transfer [51] among others.For those various applications, as the surface oxidation ofliquid metal drastically increases the wettability, the super-lyophobic surface against liquid metal has been of interest.

Super-lyophobic surfaces against mercuryThe most well-known liquid metal is mercury. The surfacetension and contact angle of various substrates againstmercury have been studied from 1920s. The reported sur-face tension of mercury is in the range of 400 ~ 516 mN/m[52] in air environment at room temperature. The typicallyacceptable value of surface tension is ~ 480 mN/m [53]which is much greater than that of water (72.9 mN/m).Therefore, the contact angle of mercury is much higherthan that of water on the same solid substrate based onYoung’s equation [9].As the contact angle study was investigated without

development of micromachining technology, most studiedsubstrates were simply flat surface but treated with differ-ent chemicals such as low surface energy material likeTeflon (surface energy ~ 18.5 mN/m). Yarnold reportedthat the advancing and receding contact angle of mercuryon the steel which was washed in ether were 165° and130°, respectively [54]. Gray also reported dynamic contactangle of mercury on low-energy solid substrates suchas polyethylene, paraffin wax, and PTFE. The highestadvancing angle of 151.7° was obtained on the PTFE sub-strate and the lowest contact angle hysteresis of ~ 0.2° wasachieved on the paraffin wax with pure mercury by acidtreatment [55]. Ellison et al. studied the dynamic contact

angles of mercury on various substrates (tungsten, stainlesssteel, nickel, quartz, glass, and Teflon) under varyingtemperatures in the range of 25 ~ 150°C [56]. The highestadvancing contact angle of 157° was measured on theTeflon as the Teflon is ‘low surface energy’ substratecompared with other ‘high surface energy’ substrates. Inaddition, most substrates showed that contact anglehystereses were in the range of 0° ~ 3° with pure cleanedmercury which means that the mercury can be easily rolloff from the surface. The temperature effect for surfacetension of mercury was negligible. Awasthi et al. reportedthe static contact angle of mercury on graphite as 152.5 ± 2°by applying ‘spot technique’ which can be also used for hightemperature measurement, as the contact angle of liquidmetal at high temperature is useful for practical applica-tions such as metal casting, welding, and brazing [57].In particular, the contact angle of mercury on the dielec-

tric substrate was an important parameter for EWODapplications [47,48]. Shen et al. reported that the contactangle of line patterns with various contact ratio (line widthper pitch) for mercury using photolithography [47]. As thecontact ratio decreases from 1 to 0.3, the contact angleincreases from 142° to 158°. Latorre et al. reported thatthe contact angle of mercury on oxidized silicon waferwas 137° with a standard deviation of 8° [48].Recently, super-mercury-phobic surface with hierarchical

structures was demonstrated by Escobar et al. [58]. Thethermal oxidation creates pyramids with sub-micron pro-trusions resulting in hierarchical structures with micro/nano structures as shown in Figure 20. The contact angleof boron doped diamond surfaces with thermal oxidationis greater than 175° with no contact angle hysteresis(Figure 21).

Super-lyophobic surfaces against gallium-based liquidmetal alloysNon-toxic gallium-based liquid metal have been of interestlately as it has various favorable properties such as higher

Figure 17 SEM images of super-hydrophobic surface: (a-c) natural lotus leaf and (d-f) PDMS artificial super-hydrophobic surface.With permission from IEEE MEMS, Copyright 2012.


boiling point, higher thermal and electrical conductivityagainst mercury [59]. The gallium-based binary (eutecticGaIn alloy [8]) and ternary alloy (e.g., Galinstan® [59]) havebeen studied for various applications. However, it has achallenging problem which is the fact that the surface ofgallium-based liquid metal alloy is instantly oxidized in airand wets almost any surfaces [60]. It is a critical issue to

have non-wetting super-lyophobic surface against gallium-based liquid metal but there was very limited report onsuper-lyophobic surface for gallium-based liquid metalalloys, probably due to the strong wettability of the gallium-based alloys.In order to prevent of wetting, Liu et al. measured, in

the below 1 ppm oxygen environment, the advancing and

Figure 18 Water static contact angles of the artificial super-hydrophobic surfaces. With permission from IEEE MEMS, Copyright 2012.


receding contact angle of pure Galinstan® on the varioussubstrates such as tungsten, silicon nitride, glass, parylene,Teflon®, phlogopite, and muscovite. It was found thatGalinstan® was non-wettable on all surfaces under thecondition. Among them, the muscovite showed the high-est advancing of 163.6° and receding contact angle of148.1°, resulting in the smallest contact angle hysteresis of15.5°, as the substrate has higher surface roughnesscompared to others [59].Oxide-free gallium-based liquid metal alloys are crucial

for certain applications like micro switches. However, inother applications such as micro-cooling and FSS, main-taining true liquid phase of gallium-based liquid metalalloy is not necessary. Recently, we measured staticcontact angle of oxidized Galinstan® on a glass, Cytop, and

Figure 19 Optical images of 10 μL water droplet on the broccoli like arCopyright 2012.

Teflon, in air environment [61]. Among them, Teflonsubstrate shows highest contact angle of 140.3°. However,the contact angle of the Galinstan® droplet was increasedto be 152.5° by removing oxide layer using hydrochloricacid vapor.In addition, we tested efficacy of the hierarchical

micro-/nano-scale super-hydrophobic thin film formationtechnique to gallium-based liquid metal alloy. Withunder-baked and under-exposed photoresist mold asexplained in section 3.2.4, PDMS multi-scale surfaceextured micro pillar array was fabricated. We found theCassie and Wenzel states as well as transition state asshown in Figure 22 [62]. In addition, we studiedlyophobicity of various pitch distance in the range of50 ~ 525 μm micro-/nano-scale hierarchical micro pillar

tificial super-hydrophobic thin film. With permission from IEEE MEMS,

Figure 20 Effect of thermal oxidation on the topography of the crystals. (a–d) AFM profile of an 60 × 60 μm2 portion of the microcrystallinefilm (a, c) before and (b, d) after oxidation. The depth of the features increases by about 1.5 μm. (e, f) SEM images of (e) microcrystalline diamondsurface, and (f) after thermal oxidation showing the transformation from flat crystals to pyramid-like structures with submicron-size tops. Withpermission from Applied Surface Science, Copyright 2013.


array by measuring contact and sliding angels (Figure 23).By conjunction with study of contact images on variouspitch distance micro pillar arrays, the contact angles ofmicro pillar arrays with pitch distance < 275 μm (in Cassiestate) were higher than those of the micro pillar arrays

Figure 21 Contact angle of mercury droplet. Contact between dropletsthem (d, e, f). The columns correspond to (a, d) the polished singe crystal

with pitch distance > 275 μm (in Wenzel state). Inaddition, the sliding angle could be only obtained onmicro pillar arrays with pitch distance < 275 μm (in Cassiestate). The highest static contact angle of 163° and thelowest sliding angle of 17.4° were achieved on the 175 μm

of mercury resting on the surfaces (a, b, c) and being detached from, (b, e) the micro-crystalline diamond and (c, f) the oxidized sample.

Figure 22 7.8 μL oxidized Galinstan® droplet on (a) 175 μm pitch pillar array, forming a non-wetting Cassie state (air pockets), (b) 275 μmpitch pillar array, on which the droplet partially fill in surface texture (air pockets and no air pockets area), and (c) 525 μm pitch pillar array(no air pockets), where the droplet fully wets the surface texture. With permission from IEEE MEMS, Copyright 2012.

Figure 23 Contact and sliding angle as a function of pitch distance for 7.8 μL oxidized Galinstan® droplet. With permission from IEEE MEMS,Copyright 2012.



pitch distance pillar array. The result clearly showed thatthe high contact angle and low sliding angle were achieveddue to micro-/nano-scale combined hierarchical textureswith an optimum pitch distance.

ConclusionsRecent progress on the hierarchical micro-/nano-scalestructures for the artificial super-hydrophobic surfaces andsuper-lyophobic surfaces against gallium-based liquid metalalloys are reviewed. As one of the key contributing factorsfor the natural super-hydrophobic surfaces is dual micro-/nano-scale surfaces, a wide variety of materials and numer-ous fabrication approaches were studied. Of those, opticallytransparent artificial super-hydrophobic surfaces are ofgreat interest in many practical self-cleaning surface appli-cations. Both SU-8 and PDMS-based super-hydrophobicself-cleaning surfaces have been demonstrated with promis-ing characteristics. As demonstrated by a few groups, it isin its utmost importance to realize super-hydrophobic andsuper-oleophobic surfaces together to be used in realapplications.Gallium-based liquid metal alloys are one of the very

interesting materials and have great potential to be utilizedin a wide variety of novel applications due to their uniqueconductive characteristic along with constantly deformableliquid characteristic. Due to its oxidation problem, it isessential to have super-lyophobic surface against the oxi-dized gallium-based liquid metal. Recently hierarchicalmicro-/nano-scale structures have been applied to super-lyophobicity surfaces against gallium-based liquid metals. Itis expected that, in the next few years, numerous un-explored potential of the liquid metal-based devices will bestudied based on recent progress of the super-lyophobicsurfaces.

Competing interestsThe authors declare that they have no competing interests.

Authors’ contributionsYY carried out research works and a survey on the super-hydrophobic surfaces.DK carried out research works and a survey on the liquid metals. JL supervisedall research works and writing of the manuscript. All authors read and approvedthe final manuscript.

AcknowledgementThe authors would like to thank Dr. Dong-Weon Lee of Chonnam NationalUniversity and Dr. Wonjae Choi of The University of Texas at Dallas forinvaluable technical/scientific discussions and UTD clean room staff for theirsupport on device fabrication work. The authors also would like to thankRepublic of Korea (ROK) Army for financial support.

Received: 13 November 2013 Accepted: 27 May 2014

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doi:10.1186/s40486-014-0003-xCite this article as: Yoon et al.: Hierarchical micro/nano structures forsuper-hydrophobic surfaces and super-lyophobic surface against liquidmetal. Micro and Nano Systems Letters 2014 2:3.

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